Strategy feedback request: 3000 sq ft Rocket Mass Heater

Hi everyone,
My father has an old brick building, that used to be heated by a oil / wood combo.
The heat exchanger has a crack and the oil barrel is too old, so the unit is non functional any more.
I would like to removed the furnace and make a rocket mass heater, as it's on the ground floor (old cement on grade, no basement).

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Does anyone have experience with parts I might want to salvage from the old furnace?
Can anyone recommend a design I might want to start exploring? The space between two old chimneys is about 10' x 10' x 10'  (used to be a blacksmith shop circa the 1900s)

Hoping to keep the 3000 sq ft building above freezing this winter. (not insulated)



I know there are some books on the topic, as well as a guide here on Permies.
However, I figure someone with experience might be able to give me some tips regarding the following:

I would keep the door, the metal casing of the furnace.

Check this thread. With all of it's pics, and the evolution of the thing
https://permies.com/t/44806/Cobbling-workshop-heater-cooktop-oven

If this can be of any help. It heats up my workshop, and my flat above. 635 cubic meters. poor insulation, and good -20 celcius outside in the winter. Never drops under 60F°. Mass is 4 tons.

Any firebrick would also be good to keep. Refractory firebox liners could also be useful as long as they are not asbestos. No furnace from recent decades will have asbestos, but I don't know about really old ones. If you find that there is asbestos in liners or ductwork wrappings, take care if you need to remove it. At the very least make sure it is wet before handling and wear full appropriate safety gear. If you can afford professional remediation it would probably be worth it for peace of mind.

Depending on the actual shape and condition of firebox liners, they may be useful in building your new rocket combustion core.

For a design in that big space, I would go with a batch box, likely at least 8" size, maybe larger depending on your climate and how warm you want to keep the building. Is the 3000 sf per floor, or total? How much do you want to heat the upper floor? Can you take out a small bit of floor over the RMH area?

I would recommend a masonry bell (or two) as the most effective method for the space: a tall hollow brick box with the exhaust coming from the bottom of the hollow space so nearly all the heat is absorbed by the brick before going to the chimney. Lots of information at https://batchrocket.eu/en/building#size .

What is the internal size of the chimney you want to use, and what condition is it in? An inspection would be a good idea before proceeding in a project of that size.